A common requirement for analog signal processing applications, such as telecommunications, is filtering. A filter alters the frequency components in a signal. For example, an analog signal may include both desired and unwanted signal components. Typically, the desired and unwanted components occupy different portions of the frequency spectrum. A filter removes the unwanted component by rejecting the signal components outside of the desired frequency band. If the frequency band of the unwanted signal is higher or lower than that of the desired components, a lowpass or highpass filter, respectively, may be used to reject the unwanted components. If the desired components occur only within a particular range of frequencies, then a bandpass filter removes unwanted components having frequencies both above and below the range of frequencies.
The availability of operational amplifiers fabricated on integrated circuits with nearly ideal characteristics simplifies filter design. Several filter types based on operational amplifiers may be used, each with its own advantages and disadvantages. For instance a simple, continuous-time filter may be constructed with an operational amplifier and additional resistors and capacitors. The values of the resistors and capacitors are chosen so that the filter has the desired frequency response. This filter type has the advantage that it provides a continuous output signal. However, resistors and capacitors formed on integrated circuits have very wide tolerances. Furthermore, the use of accurate, discrete resistors and capacitors external to the integrated circuit presents additional costs for a circuit board designer. Thus, in many applications, a discrete filter using an operational amplifier and resistors and capacitors is undesirable.
Other filter designs are available, but each presents its own problems, especially if continuous-time filtering is required. A switched-capacitor filter uses digital switching and clocking techniques and provides a frequency response based on the ratio, rather than the absolute value, of capacitors. Thus, the switched-capacitor filter is not sensitive to the wide tolerances of integrated circuit components. However, switched-capacitor filters have their own problems. In order to provide a continuous-time filter, an additional smoothing filter is needed at the output of the switched capacitor filter. A switched-capacitor filter also suffers from performance degradation for higher clock frequencies.
Another type of filter is called an operational transconductance amplifier-capacitor (OTA-C) filter. An OTA-C filter, however, is inherently nonlinear, because its transfer characteristic is typically based on either an exponential (bipolar transistor) or square-law (MOSFET) amplifier. While OTA-C filters are tunable, they have a poor dynamic range. Thus, a continuous-time, linear filter which remains accurate even under wide component tolerances is needed.